1. Field of the Invention
The present invention relates to a current detection circuit of a bidirectional switch which monitors a charge current and a discharge current of a direct current power source such as a rechargeable battery, and protects the direct current power source by cutting off any excessive charge current and any excessive discharge current.
In the drawings, the same reference numerals and signs denote the same or equivalent parts.
2. Description of Related Art
FIG. 3 is a diagram showing an example of a configuration of the simplest related current detection circuit of such a kind. In the diagram, a MOSFET Q1, acting as the main switch, carries out on-and-off switching of the flow of a load current Im, which is the object of detection. In series to the MOSFET Q1, a high-accuracy sensing resistor Rs is inserted. A voltage across the sensing resistor Rs is observed in order to detect the current Im. In this circuit, however, when the current Im to be detected is particularly large, the large amount of power lost in the sensing resistor Rs becomes a problem. In practice, the gate of the MOSFET Q1 is attached to a control circuit (not shown).
FIG. 4 illustrates a device for detecting a load current Im with low power loss. In the device shown in FIG. 4, a current mirror circuit is formed by providing two N-channel MOSFET's Q1 and Q3. The N-channel MOSFET Q1 acts as a main switch, in which the load current Im flows. The N-channel MOSFET Q3 acts as a mirror switch. It has a gate potential made equal to that of the N-channel MOSFET Q1 and has its W (channel width) length being 1/n of that of the MOSFET Q1. In this mirror circuit, a negative feed back circuit is provided that uses an element with a high input impedance (in the example, an operational amplifier Op1) so that a voltage across the N-channel MOSFET Q1 and a voltage across the N-channel MOSFET Q3 are made equal to each other. In the negative feed back circuit, a sensing resistor Rs is inserted between the output terminal of the operational amplifier Op1 and the drain of the N-channel MOSFET Q3 to cause a mirror current Is to flow with an exact magnitude Im×(1/n) in the sensing resistor Rs. The ratio can be the result of the physical dimensions of the MOSFET's, e.g., the ratio of widths W.
The mirror current Is is sufficiently small, compared with the load current Im, to cause a very small loss when being detected. The mirror current Is is accurately detected by an element with a low input impedance (in the example, an operational amplifier Op1) and the sensing resistor Rs, by which the current Im in the main FET Q1 can be accurately detected.
In the current detection systems shown in FIGS. 3 and 4, the direction of the current Im flowing in the main switch is fixed. Thus, with these systems, it is impossible to detect a current flowing in either direction, to provide a charge and discharge protection switch.
FIG. 5 is a diagram showing an example of an ordinary configuration of the main switch section of a charge and discharge protection switch (bidirectional switch) for cutting off an excessive charge current, and also an excessive discharge current, of a battery E. In the diagram, two main switches M1 and M2 are inserted in a charge/discharge current path on the side of the negative terminal of the battery E. Each of the switches M1 and M2 is made of an N-channel MOSFET, which is a device that naturally contains a parasitic (or, inherent) diode (D1 and D2 respectively). In the illustrated example, the inserted main switches M1 and M2 are connected in series with their polarities reversed to each other, so that the source terminals of the main switches M1 and M2 are at both ends of the series connection.
Here, with the main switch M1 driven to be turned-on and the main switch M2 driven to be turned-off, a charge current of the battery E can be cut off; and, with the main switch M1 driven to be turned-off and the main switch M2 driven to be turned-on, a discharge current of the battery E can be cut off.
An example of a detection system for current flowing in either direction in such a bidirectional switch is disclosed in JP-A-11-69635. This document discloses a circuit configuration in which a sensing resistor inserted in a current path is changed to a sensing resistor with a value suited for current detection, depending on the magnitude of a charge current or a discharge current flowing in the current path. In this circuit, a voltage across the sensing resistor is detected by an operational amplifier so that an accurate current detection voltage is obtained with little influence of the offset of the operational amplifier, while reducing power loss in the sensing resistor.
Also, in JP-A-2003-215172, a charge and discharge current detection circuit is disclosed which enables detection of a charging current and a discharge current to be carried out under the same operating condition. In the detection circuit disclosed in this document, characteristics factors of an amplifying circuit are made to similarly affect the detection of the charging current and detection of the discharge current, to enable accurate comparison of them with easy offset adjustments.
The system disclosed in JP-A-11-69635, however, involves a problem, in that the value of the sensing resistor must be selected according to the current output specifications of the device for reducing a power loss in the sensing resistor and increasing the sensing voltage across the sensing resistor. Furthermore, in the system in JP-A-11-69635, there are some losses caused in the sensing resistor inserted in the charge and discharge current path, which still remains as a problem.
It is an object of the present invention to solve the problems described above and to provide a current detection circuit of a bidirectional switch that can accurately detect a charge current and a discharge current with low power loss, by a simple circuit.